Just upgrade to Big Sur. Can't find a way for a "FLAG" to show up next to "FLAGGED" message???

While in Mail application the top Menu Bar >>" View " >> pop down menu >> " Customize Toolbar "

That should present much like in image below

If the Flag icon in not present is your original Toolbar >> drag the Flag icon into it

That is going to Depend if the user is using the Default View

First image Default View the Flag is on Right side

If the user is using the " Column View " the flag is on Left Side Second Image

If using Column View and Flags are not showing at all

Click the Header " Subject " and sub menu appears and Enable Flags Third Image

You are welcome and Thank you too 🇨🇦